A variety of ion sources are available for generating an ion beam. Types of ion sources include electron impact ionization (EI), chemical ionization (CI), glow discharge ionization (GDI), fast atom bombardment, matrix-assisted laser desorption ionization (MALDI), Thermal ionization (TIMS), Secondary ionization (SIMS), Selected Ion Flow Tube and Plasma source. EI and CI are commonly used for gas phase analysis by most commercially available instruments. GDI as disclosed by McLuckey et al. is exclusively for gas phase analysis. The arrangement disclosed by McLuckey et al. provides better sensitivity as compared to the sensitivity typically achieved by commercial instruments (1 pg) using standard electron impact (EI) and chemical ionization (CI) ion sources [See McLuckey, S. A., Goeringer, D. E., Asano, K. G., Vaidyanathan, G. and Stephenson, J. L. (1996). “High explosives vapor detection by glow discharge ion trap mass spectrometry.” Rapid Communications in Mass Spectrometry 10:287-298].
The schematic of a conventional atmospheric sampling glow discharge ion source system 100 according to McLuckey et al. is shown in FIG. 1(a). System 100 includes a discharge chamber 101 which comprises a set of parallel plate electrodes A1 and A2. Vapor phase molecules are introduced into the discharge chamber 101 using a carrier gas. A sufficient voltage difference is applied across the electrodes A1 and A2 to induce a glow discharge between the electrodes, so that vapor phase molecules entering the discharge chamber 101 through entrance orifice 105 are ionized by the glow discharge. The typical pressure in the discharge chamber 101 of about 800 mTorr is maintained by roughing pumps which typically pump through four (4) one half inch diameter pumping ports 151-154 as seen in the top cross section view of the pumping port configuration shown in FIG. 1(b). Ions generated in discharge chamber 101 are directed out of discharge chamber 101 through exit orifice 110, such as to an ion trap mass spectrometer. Reference 115 is an Einsel lens system use to focus the ions into the ion trap mass spectrometer 120. Although 120 is shown as an ion trap mass spectrometer, it can be any type of mass spectrometer.
The ion trap portion of ion trap mass spectrometer 120 shown in FIG. 1(a) comprises a ring electrode 121, an entrance end cap electrode 122 and an exit end cap electrode 123 placed opposite each other with the ring electrode 121 between them forming an ion trap space surrounded by the ring electrode 121, the entrance end cap electrode 122 and the exit end cap electrode 123. The electronics associated with ion trap 120 include a primary RF voltage generator for applying an RF voltage to the ring electrode 121 to trap object ions of a predetermined mass-to-charge ratio, and auxiliary voltage generator for applying an auxiliary AC voltage to the end cap electrodes 122 are not shown for simplicity.
FIG. 2 shows the tandem mass spectrum (MS2) obtained from the injection of 500 fg of RDX explosive into an oven (not shown) located directly in front of the entrance orifice 105 on electrode A1 in FIG. 1(a). The inset of FIG. 2 shows the total ion chromatogram signature as the RDX vapor diffuses into the discharge chamber 101. It is noted that the GDI process is gentle enough to limit dissociation to yield parent ions, even for explosives.
Each point in the chromatogram shown represents a mass spectrum. Consequently, the entire 500 fg sample is spread out over approximately 20 mass spectra. The signal to noise ratio in the MS/MS spectrum is approximately 30. Therefore, the estimated analyte sensitivity is approximately 10 fg provided the entire sample is injected into the discharge all at once as would be the case for particle injection into the discharge chamber 101.
The sensitivity of conventional atmospheric glow discharge ion sources, such as the ion source shown in FIG. 1(a), is limited by pumping away of analyte via the four (4) half inch diameter roughing ports 151-154 used to maintain the discharge chamber pressure between about 0.1 to 1.0 Torr. The vast majority of the gaseous analyte admitted through the entrance orifice 105 in electrode A1 into discharge chamber 101 is removed and thus cannot be analyzed when it is pumped out through the roughing ports, as opposed to the desired path where it passes through the typically 200-μm diameter exit orifice 110 in electrode A2 into the ion trap mass spectrometer 120 for detection. A rough estimate of the ratio of analyte exiting through the pumping ports to the analyte sampled into the main chamber can be obtained from the ratio of the total cross sectional area of the pumping ports to the area of the exit orifice 110 at electrode A2, which is approximately 16,000.